---
title: Removed req.ua from Middleware API
---

## Why This Error Occurred

Your middleware is interacting with `req.ua` and this feature needs to opt-in.

```ts filename="middleware.ts"
import { NextRequest, NextResponse } from 'next/server'

export function middleware(request: NextRequest) {
  const url = request.nextUrl
  const viewport = request.ua.device.type === 'mobile' ? 'mobile' : 'desktop'
  url.searchParams.set('viewport', viewport)
  return NextResponse.rewrite(url)
}
```

## Possible Ways to Fix It

To parse the user agent, import `userAgent` function from `next/server` and give it your request:

```ts filename="middleware.ts"
import { NextRequest, NextResponse, userAgent } from 'next/server'

export function middleware(request: NextRequest) {
  const url = request.nextUrl
  const { device } = userAgent(request)
  const viewport = device.type === 'mobile' ? 'mobile' : 'desktop'
  url.searchParams.set('viewport', viewport)
  return NextResponse.rewrite(url)
}
```